It's been awhile, but I seem to recall that the ALS-1300 runs off two 48VDC power supplies (in a single cabinet) - each power supply powering half of the amp. If this is correct, I am not sure how easy it would be to power it from a single 48V battery supply.

I should have been a little more specific in my initial post, so here are some more details:

1. The other voltages involved won't be a problem (+12V from the "lowest" battery in the chain; -12V from some kind of DC/DC converter.)

2. As for the split power supply feed, my initial thought is that this was done more for the convenience of Ameritron, in not having to source a smaller quantity of (presumably more expensive) SMPS modules, so my assumption (untested) is that the two busses could be connected when using an external battery power source.

3. My still-unanswered concern is actually what output transistor drain over-current protection (if any) the standard Ameritron switching power supply supply affords -- somewhere in the manual of the ALS-1300 I saw that it mentioned "current limited" power supply or something to that effect. The schematics (of the amplifier) don't seem to have any over-current protection (or I overlooked that.) I remember from my old Ten-Tec Argosy 525 that Ten-Tec advised use of a 9-amp DC circuit breaker if you were going to run that unit from battery (vs. their AC power supply which I presume had some current limiting or comparable feature.)

So, updating my question: What risks (otherwise mitigated by use of Ameritron's power supply) to the output transistors does operation from (high current) batteries pose?

The power supplies will need to be sequenced correctly. The +12/-12V supplies must be up before the +48 V supply begins to ramp up.

In the ALS-1300 the power supplies ramp up over several milliseconds. So, switching the voltages ON with a relays might not be a good thing. That would pound the ALS-1300 with high surge current and overvoltage ringing.

I would use MOSFET switches that ramp up the current over a period of tens of milliseconds. And a current sense that will shut the +48 VDC path OFF would be desirable. If you would like to use relays step-start resistors can be used to soften the inrush current.

If you need a circuit designed to do this I can do it. BTW, I have an ALS-1300.

Offer to design circuit appreciated! For now, a few more/related questions:

One other generic question I forgot to include --- at full charge, the "12V" batteries would actually be ~14V (they'd be on float charge), presenting ~56V to the ALS-1300. Presumably (but not guaranteed) the bus voltage would drop off this value as soon as the ALS-1300 is told to transmit. Would this be a source of concern?

Do you think the MOSFETs you propose could be brought up in a controlled manner to accomplish step-start effect without actually having large resistors in the path? That might reduce the number of connections in the high-current path...

Would there be any drain current from the ALS-1300 (other than charging bypass capacitors, etc.) if the +/-12 starts first (put another way, would the ramp-up phase be at relatively low current so the MOSFETs wouldn't be called on to dissipate much current while in the linear phase?)

Would some kind of interlock be needed (or is there one already included) to prevent the ALS-1300 from being commanded into transmit mode until all of the supply voltages are stable?

Are there suitable (= at a reasonable price) MOSFETs with (saturated) Rds On low enough to avoid significant heat dissipation in those parts once they're turned on?

Sounds to me like an accident waiting to happen. With batteries of high current capacity, any short will cause a good deal of destruction and maybe a fire. If you intend to use battery power to run the amp then using a switching supply powered by a bulk supply (batteries) will be much safer. You could even use the regular supplyoperated by a battery bank and an invertor to provide 115 VAC.You really should outline what you hope to accomplish so that the reader can betterunderstand your plan.

My ALS-600 with the linear power supply runs about 55 volts unloaded. This is what the ALS-1200 on batteries would see and it should be ok. If this is a concern a series pass regulator can be built that is set to 50 volts. When the batteries drop below 50 volts it will act as a hard switch.

During ramp-up the MOSFET must dissipate energy equal to the energy stored in the ALS-1300 capacitors. The capacitance is 200 uF and the energy is only 0.25 joules. This is fine for even a TO-220 package FET. The FET ON resistance should be 5 milliohms to limit the voltage drop to 0.25 volts. The PA draws zero drain current as the 50 volt rail is ramping up.

The switch can be an N-channel FET with a high side switch IC or a P-channel FET with the gate pulled towards ground. The drawback to the high side switch is that all the high side switch ICs are surface mount, as far as I know. I can take a look. The drawback to a P-channel FET is 3X more silicon is used for the same ON resistance and that means more cost and paralleling FETs. Paralleling FETs invites oscillation when in the linear region. A perfectly symmetrical layout helps avoid this. I have learned this the hard way. So let's avoid paralleling FETs.

Yes an interlock can be built to holdoff the ALS-1300 keying input until the power supplies are stable. If any supply drops out of range the interlock opens.

I would use one N-channel MOSFET per PA. The IRFP4368PBF, 75V, 195A, TO-247-3 package, 1.85 milliohm looks good. Digikey has them in stock at $8.79. The power dissipation key down is 1.3 watts per FET and a heatsink is not needed. A high side gate driver can be built.

This is not a trivial design but as I design power electronics, and have designed such circuits, it will work as designed and be relatively simple to build.

An alternative to rolling your own is to purchase a 48 VDC to 230 VAC inverter to power the ALS-1300.

It did occur to me that directly combining the two DC rails probably isn't such a good idea, as catastrophic failure in one of the halves of the RF deck would subject that part of the internal high current wiring to the ~50A fuse needed to protect the combined batteries, and likely the internal wiring was sized for 25A max current. Secondary fusing at 25A into each half should take care of that concern.

Would sure be nice if MFJ just built a DC battery adapter product for this amplifier, vs. homebrewing one. Although the homebrew project sounds quite doable with your detailed description and offer of design help, it is a one-off project which would compete for time among other projects now underway. Still, your road map seems clear...

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